1. Field of the Invention
This invention relates generally to a system and method for controlling the cathode stoichiometry in a fuel cell and, more particularly, to a system and method for controlling the cathode stoichiometry in a fuel cell that includes maintaining the cathode stoichiometry applicable for high fuel cell power for a predetermined period of time after transitioning to low fuel cell power and maintaining the cathode stoichiometry applicable for low fuel cell power for a predetermined period of time after transitioning to high fuel cell power so as to reduce relative humidity excursions.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). The membranes within a fuel cell need to have a certain relative humidity, such as 80%, so that the ionic resistance across the membrane is low enough to effectively conduct protons.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include two hundred or more fuel cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
The fuel cell stack includes a series of flow field or bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode reactant gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the anode side of the MEA. Cathode reactant gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of the MEA. The bipolar plates also include flow channels through which a cooling fluid flows.
Cathode stoichiometry is directly proportional to the ratio of the volume of cathode input air applied to the stack to the current density generated by the stack. A typical cathode stoichiometry is about 2 for relatively high fuel cell stack current densities. When the output of the stack goes to low power, such as during an idle condition, it is known in the art to increase the cathode stoichiometry so that the fuel cell stack remains stable. Particularly, accumulation of water within the reactant gas flow channels from water by-product could cause the cells to fail because of low reactant gas flow, and thus affect the stack stability. A voltage cell potential less than 100 mV is considered a cell failure. The volume and rate of input air for the cathode stoichiometry for normal fuel cell operation at low stack loads is not great enough to force the water out of the reactant gas flow channels. Thus, the cathode stoichiometry is sometimes increased at low stack power to increase the airflow and hence stack stability. Consequently, the operating temperature of the fuel cell stack needs to be decreased when the cathode stoichiometry is increased to reduce the membrane drying effect from the increased airflow to maintain a desired membrane relative humidity.
FIG. 1 is a graph with stack current density on the horizontal axis, cathode stoichiometry on the left vertical axis and temperature on the right vertical axis showing the relationship between stack power, cathode stoichiometry and fuel cell temperature discussed above. Graph line 10 shows the relationship between cathode stoichiometry and stack current density and graph line 12 shows the relationship between stack temperature and stack current density. From a stack perspective, a sustained low power stoichiometry of about 1.8 causes instability as a result of water accumulation in the reactant gas flow channels, and an increased stoichiometry to 4 or 5 results in better stability. Consequently, to maintain a desirable membrane relative humidity, the operating temperature of the stack needs to be reduced. From a system perspective, it is difficult to maintain elevated temperatures at sustained low power due to heat losses.
As discussed above, in some known fuel cell systems, as the cathode stoichiometry increases, the controller reduces the temperature of the stack. However, the temperature response time is limited by slow thermal dynamics relative to the change in stoichiometry. In other words, the temperature does not reduce fast enough. This mismatch in thermal dynamics causes the relative humidity of the membrane to be reduced to about 50%. Similarly, as the load increases and the cathode stoichiometry is reduced, the stack temperature is increased. However, the mismatch between airflow and thermal dynamics causes the relative humidity to increase beyond 120% and decay back to the desired 80% when the temperature responds. Thus, unwanted relative humidity excursions occur at load transients.